A histological study of the sweat glands of normal and dry-coated horses

Genomic Association of Chronic Idiopathic Anhidrosis to a Potassium Channel Subunit in a Large Animal Model

Similar to humans, the horse relies predominantly on the evaporation of sweat from the skin surface to dissipate excess body heat. Loss of the sweat response or anhidrosis can result in life-threatening hyperthermia. Anhidrosis occurs more frequently in some breeds as well as occurs at an increased frequency among individuals with a family history, suggesting a heritable component to the pathology. Given the natural occurrence and indications of genetic components in the etiology, we utilized genomics to better understand the molecular mechanisms involved in sweat response. We performed a case-control (n = 200) GWAS targeting cases of chronic idiopathic anhidrosis in a controlled genetic background to discover the contributing regions and interrogated gene function for roles in the sweating mechanism. A region containing the KCNE4 gene, which encodes the β-subunit of a potassium channel protein with a possible function in sweat gland outflow, was associated (P = 1.13 × 10^-07) with chronic idiopathic anhidrosis through GWAS. A candidate mutation (NC_009149.3:g.11813731A > G, rs68643109) disrupting the KCNE4 protein structure could explain the disease but requires further investigation in larger populations. We show the potential role of ion channels and cellular damage in sweat response, correlating anhidrosis as a possible effect of congenital channelopathy.

Seasonal influences on quantitative changes in sweat-associated anatomy in native and thoroughbred horses

Stereological techniques were used to assess seasonal influences on morphometric characteristics of hair follicles, sweat and sebaceous glands in abattoir pelts of ponies (PN), thoroughbred (TB) and non-thoroughbred (NTB) horses. Volume density of sweat glands increased significantly from winter (0.061) to summer (0.098) in TB, and showed no change in NTB and a positive tendency in PN. There might be a body surface area : volume effect for sweat gland parameters as PN had smaller values than either TB or NTB, probably attributable to control of heat loss in winter. In summer, the skin remained thick and the volume density of sebaceous glands was increased in NTB, in contrast to TB where both were decreased. It is possible that in summer, sebum has a particular importance in NTB to enhance wicking of sweat through the pelt. TB showed significantly higher volume measurements of sebaceous glands than NTB and PN for winter: sebum has probably a special importance for water-proofing in TB in winter. PN showed no significant seasonal changes in sebaceous glands, but had a thinner summer skin. Winter values for hair follicle volume density between equine groups were similar (TB, NTB 0.066; PN 0.059), as was skin thickness (1.14-1.19 mm). The volume density lowered significantly in summer in TB and NTB. The volume of hair follicles under a unit area of skin surface decreased significantly in TB and nonsignificantly in NTB and PN. The seasonal adaptations of the skin shown here were most pronounced in TB and differed between breeds.

The absence of apoeccrine glands in the human axilla has disease pathogenetic implications, including axillary hyperhidrosis

BACKGROUND

The existence of a third type of sweat gland in human axillary skin, the apoeccrine gland, with a capacity to produce much higher sweat output than the eccrine gland, was proposed from examination of microdissected glands. However, previous studies of axillary skin glands did not examine the entire individual glandular structure via serial sections and the markers used to identify the different glands gave conflicting results and, hence, the existence of the apoeccrine gland remains controversial.

OBJECTIVES

To investigate human axillary sweat glands by serial section histology and immunofluorescence.

METHODS

Human axillary sweat glands were investigated by serial sectioning of paraffin wax-embedded skin samples taken by biopsy from four male and six female volunteers (age range 20-35 years). Sections were examined by light microscopy and immunofluorescence, using antibodies to antigens reported to be markers for discriminating between eccrine and apocrine gland cells: CD15, CD44, S100 and human milk fat globulin.

RESULTS

Light microscopy demonstrated that there were hair follicles and a mean +/- SD of 76 +/- 14 sweat glands cm(-2). Eccrine and apocrine glands were found to be present; however, no glands resembling the apoeccrine glands were detected. Both types of sweat gland exhibited signs of being active, with segments of the secretory coils displaying flattened cells and dilated glandular lumina; however, this dilation did not extend to obvious changes in the width of the gland. None of the eccrine glands exhibited evidence of the presence of apocrine cells or vice versa. Immunofluorescence markers were found not to be specific and did not discriminate between the different types of glands or demonstrate the presence of apoeccrine glands.

CONCLUSIONS

This is the first time that serial sections of axillary skin have been examined by histology and immunofluorescence. The markers reported to discriminate between apocrine and eccrine glands were found to be nonspecific. No evidence of apoeccrine glands was found either by histology or by immunofluorescence.

A preliminary study of the short circuit current (Isc) responses of sweat gland cells from normal and anhidrotic horses to purinergic and adrenergic agonists

The causal factors of equine anhidrosis have not yet been elucidated but defective electrolyte transport mechanisms in the gland are likely to be involved. To investigate this possibility, experiments were performed on cultured equine sweat gland epithelia from five free-sweating UK horses (3 intact males, 2 mares, aged 2-4 years) and from three free-sweating Singapore horses (1 intact male, 2 mares, aged 3-5 years) and three anhidrotic (Singapore) horses (1 intact male, 1 gelding, 1 mare, aged 3-6 years). Cultured cells from each animal were grown on permeable supports and loaded into Ussing chambers to quantify transepithelial resistance and agonist-induced electrolyte transport by the short circuit current (Isc) technique. Transepithelial resistances across the layers of cultured cells were not significantly different between cells from UK and Singapore free-sweating horses, but were significantly reduced in anhidrotic animals. Purinergic agonists added to the apical and basolateral aspects of the cultured cells caused similar increases in Isc between the two populations of unaffected cells, but Isc increases were significantly reduced in anhidrotic animals. Beta-adrenergic agonist stimulation of the anhidrotic cell layers failed to elicit any change in Isc. These pilot results not only confirm earlier conclusions from anatomical findings that failure in the secretory process occurs in anhidrosis but also indicate that both of the known ion transport mechanisms are involved. The trigger for these failures warrants further investigation.

Equine sweating and anhidrosis Part 2: anhidrosis

The condition of anhidrosis is described in this review, and the latest theories on the causal factors are explored. The evidence supports the hypothesis that anhidrosis is an inappropriate response to prolonged climatic stress (generally combined heat and high humidity), which can be evoked in a small (approximately 10 +/- 5%) proportion of the equine population. It is caused by gradual failure of the glandular secretory cell processes, initiated by desensitization and subsequent down-regulation of the cell receptors as a result of continued adrenaline-driven hyperactivity. It progresses through secretory failure and culminates in gradual, probably irreversible, glandular dedifferentiation and ultimate degeneration. There is a need for considerably more research on the secretory and transcriptional processes to document the changes arising within the glandular secretory mechanism as a prelude to development of a corrective treatment.

Equine sweating and anhidrosis Part 1 ? equine sweating

Sweating has a variety of functions in mammals including pheromone action, excretion of waste products and maintenance of the skin surface ecosystem. In a small number of mammalian species, which includes humans and the Equidae, it also has an important role in thermoregulation. This review is focused specifically on the thermoregulatory role of sweat in Equidae and the causes of sweating failure (anhidrosis). The first part describes the glandular appearance, sweat composition, and output rates; and considers the latest theories on the glandular control and secretory mechanisms. It is concluded that the glands are not directly innervated but are controlled by the interplay of neural, humoral and paracrine factors. The secretory mechanism is not as simple as previously thought and is mediated by the dynamic interaction of activating pathways, including autocrine control not only of the secretory process but probably also of secretory cell reproduction, growth, and death.

Immunolocalization of aquaporin-5 expression in sweat gland cells from normal and anhidrotic horses

Western blot analysis showed that sweat gland cells from freely sweating horses expressed the water channel aquaporin-5 (AQP-5). Immunohistochemistry revealed a strong AQP-5-like activity reaction at the apical membrane of the glandular secretory cells, which was absent from the surrounding myoepithelium and all other skin structures. In anhidrotic horses, AQP-5 was also found at the apical membrane of the luminal sweat gland cells, but the level of expression reduced with the length of time that the animal had displayed anhidrosis. The level of AQP-5 expression was substantially reduced in animals with long-term anhidrosis, hence implicating water channel impairment as a possible factor in the development of this disorder.

Cytochemical characterization of glycoconjugates in the apocrine glands of the equine scrotal skin

Cytochemistry of glycoconjugates in the apocrine glands in the scrotal skin of the horse was studied using cytochemical methods for electron microscopy, particularly lectin cytochemistry. The secretory cells possessed a variable number of secretory vesicles, a well-developed Golgi apparatus, and abundant cisternae of the rough endoplasmic reticulum. Additionally, the basolateral plasma membrane formed numerous interdigitating folds. Glycoconjugates with vicinal diol groupings were present predominantly in the secretory vesicles, the Golgi apparatus, the surface coat of the plasma membrane, and the majority of the intracellular membranes. With lectin cytochemistry, the secretory vesicles of the glandular cells exhibited glycoproteins with different terminal sugars (alpha-D-mannose, beta-D-galactose beta-N-acetyl-D-glucosamine, and sialic acid). Several sugars were distinctly prominent in the surface coat of the plasma membrane of the secretory cells. The cytochemical properties of the complex glycoconjugates found are discussed in relation to the specific functions of the glandular secretions. These glands may have an important role in not only thermoregulation but protection of the scrotal skin, a specific body region.

Acute effects of dehydration on sweat composition in men during prolonged exercise in the heat

AIM

To determine whether acute exercise-heat-induced dehydration affects sweat composition, eight males cycled for 2 h at 39.5 +/- 1.6% VO2peak on two separate occasions in a hot-humid environment (38.0 +/- 0.0 degrees C, 60.0 +/- 0.1% relative humidity).

METHODS

During exercise, subjects ingested either no fluid (dehydration) or a 20 mmol L(-1) sodium chloride solution (euhydration). The volume of solution, calculated from whole-body sweat loss and determined in a familiarization trial, was ingested at 0 min and every 15 min thereafter. Venous blood was collected at 0, 60 and 120 min of exercise and sweat was aspirated from a patch located on the dominant forearm at 120 min.

RESULTS

Following the 2-h cycling exercise, sweat [Na+] and [Cl-] was greater (P < 0.05) in the dehydration trial (Na+ 91.1 +/- 6.8 mmol L(-1); Cl- 73.3 +/- 3.5 mmol L(-1)) compared with the euhydration trial (Na+ 81.1 +/- 5.9 mmol L(-1); Cl- 68.5 +/- 3.3 mmol L(-1)). In addition, dehydration invoked a greater serum [Na+] (142.2 +/- 0.7 mmol L(-1); P < 0.05), [Cl-] (105.8 +/- 0.6 mmol L(-1); P < 0.05) and [K+] (5.27 +/- 0.2 mmol L(-1); P < 0.05) over the euhydration values for [Na+], [Cl-] and [K+], respectively (138.9 +/- 0.6, 102.9 +/- 0.5 and 4.88 +/- 0.1 mmol L(-1)). Plasma aldosterone was also significantly higher during exercise in the dehydration trial compared with the euhydration trial (53.8 +/- 3.8 vs. 40.0 +/- 4.3 ng dL(-1); P < 0.05).

CONCLUSIONS

Acute exercise-heat stress without fluid replacement resulted in a greater sweat [Na+] and [Cl-] which was potentially related to greater extracellular fluid [Na+], plasma aldosterone or sympathetic nervous activity.

Equine anhidrosis

The molecular basis of the pathophysiology of anhidrosis is still not well understood. Therefore, treatments are more often based on clinical impressions than on scientific fact. Treatment options for this condition will improve only when more is known about the molecular events that cause anhidrosis, especially as they relate to beta2-receptor dysfunction and stimulus-secretion coupling in the sweat glands of affected horses. Although this additional information is being attained, sound environmental management will continue to be a very important aspect of the treatment of horses affected with anhidrosis.

Effects of acute intravenous aldosterone administration on Na(+), K(+), and water excretion in the horse

The effect of a temporary increase in plasma aldosterone concentration on Na(+), K(+), and water balance was investigated in four horses. Aldosterone was injected intravenously for 6 h at 20-min intervals (total 5.4 microg/kg body wt). Samples were taken for 24 h before, during, and for 48 h after the treatment. Aldosterone treatment reduced the Na(+) loss via urine and feces by 99 and 72%, respectively, later followed by a marked increase in Na(+) excretion by both pathways. During the first 6 h after the treatment, fecal K(+) excretion was elevated, and the plasma K(+) concentration was lowered. Fluid was retained throughout the treatment period and for 12-15 h thereafter. In a second experiment, exercise was performed once after aldosterone treatment and once without prior treatment. Sweat samples were collected, and the composition was not altered after treatment. It was concluded that acute aldosterone injections reduce Na(+) losses in both feces and urine but not in sweat. In addition, the feces was shown to be the main excretion pathway of aldosterone.

Sweat production and localisation of carbonic anhydrase in the equine sweat gland during exercise at two ambient temperatures

The aim of this investigation was to study sweat production during exercise at 2 ambient temperatures (20 degrees C and 35 degrees C) and the concurrent localisation of carbonic anhydrase (CA) in the sweat gland. Horses develop alkalosis during prolonged exercise and the sweat contains HCO3-. Carbonic anhydrase is therefore of interest since it catalyses the reaction CO2 + H2O<-->HCO3- + H+. Four standardbred trotters performed an exercise test. Skin biopsies were taken from the neck, and sweat rate, blood and skin temperatures were measured. There was a close relationship between sweat rate, temperatures and work intensity at 20 degrees C. Temperatures and sweat rate were higher at 35 degrees C and did not fall when the work intensity dropped. A significant decrease in the sweat gland cell area was found after exercise at 35 degrees C with an accompanying decrease of vesicles. Strong CA activity was present at the luminal cell membrane and weaker basolaterally. The staining intensity increased after exercise. We suggest that CA might be of importance for counteracting the alkalosis developed after exercise by delivering HCO3- for generation of the alkaline pH in sweat.

Effects of season and lower ambient temperature on the structure of the sweat glands in anhidrotic horses

Histological studies of the sweat glands of anhidrotic horses in the Hong Kong summer and under conditions of reduced thermal stress, both natural and controlled, were undertaken to determine if glandular regeneration occurs. Clinical data were collected for comparison with the histological results in each instance. Horses were assigned to one of three categories on the basis of the resulting change in the number of thin glandular profiles in a cooler environment. Group 1, which was classed as normal, had a low initial value, which was maintained. Group 2, typical of mild and moderately affected animals, had a high initial value, which fell markedly after as little as six weeks in the cool environment. Animals in Group 3, classed as severely affected, had a high initial value which remained high even after prolonged exposure to the cool environment. Light microscopical examination of the sweat glands in the heat, and after six weeks in a cool environment, provided a means of predicting the degree of anhidrotic severity and the potential for recovery in a cool climate. This was superior to clinical observation, although a diagnostic test based on glandular function is still required.

Ultrastructural variations in the sweat glands of anhidrotic horses

The ultrastructure of sweat glands from the skin of free sweating horses was compared with that of glands from anhidrotic cases. Evidence of atrophied and abnormal sweat glands in the anhidrotic horses indicates that the condition involves progressive failure of the glandular mechanism of sweat production.

Equine anhidrosis: a review of pathophysiologic mechanisms

Anhidrosis is loss of the ability to sweat. The problem is seen in horses kept in a hot humid climate, and it may cause severe impairment of thermoregulation in the equine athlete. British Thoroughbreds imported to her tropical colonies are the earliest recorded cases, and since then the syndrome has come to be described as one of Thoroughbreds, usually performance athletes, undergoing acclimatization to heat and humidity. A recent epidemiologic study of cases in Florida has shown, however, that many different breeds, and long time inhabitants of a hot climate, may be affected. Equine sweat glands are of the apocrine type, and sweating is stimulated by direct local release of epinephrine from adrenergic nerve endings and by circulating epinephrine. Lack of sweating could be due to a number of possible flaws in a sequence from central nervous stimulation through sweat stimulation and secretion to delivery of sweat to the skin surface. The most likely possibilities are inadequate sweat gland response due to habituation of receptors to a high circulating level of epinephrine and occlusion of the sweat ducts by keratin plugs. Hormonal or metabolic imbalance may play a role both in the onset and secondary signs associated with anhidrosis.

Experimental bovine Trichophyton verrucosum infection. Preliminary clinical, immunological and histological observations in primarily infected and reinoculated cattle

The cutaneous application of different doses of viable Trichophyton verrueosum to the unabraded skin of cattle of various ages resulted in clinically recognizable ringworm infection of varying extent and duration. Confluent lesions covering the whole inoculated area were produced by 10⁷viable units of the fungus, whereas the minimal infective dose of 10³ viable units produced limited areas of infection only. The level of nutrition within the limits imposed had no effect on the extent or severity of lesions. The fungus was found to invade the keratinized portions of skin and hair of cattle of all ages at the same rate. However, both the cutaneous inflammatory response and the resolution of lesions were most rapid in older animals. The ability to eliminate infection more rapidly was associated with a marked delayed hypersensitivity response commencing 14 days after infection. Such hypersensitivity was not detectable by this means after the resolution of lesions.

T. verrueosum could not be isolated in culture from skin lesions until 21 days after inoculation and could only be isolated for half the period that lesions were present. Cattle were resistant to cutaneous reinfection with viable T. verrueosum on previously infected or fresh skin sites at 2 months and at more than one year after the resolution of primary lesions. A mild delayed hypersensitivity response developed in every site within 48 hr. of reinoculation. The intravenous inoculation of previously-infected cattle with 10⁴viable units of T. verrueosum resulted in an immediate-type cutaneous reaction at the original site of infection.